HBF is a well-known approach to reduce the energy consumption of mMIMO systems through a reduction in the number of transmitting RF chains obtained by employing a hybrid structure combining a phase-shift AP with a baseband DP transmission  [1]}. As a result, HBF techniques have been considered for fifth-generation cellular networks (5G) in the mm-Wave bands, and will most likely be extended to 6G [2]}. In general, three HBF structures are considered: FC-HBF, FSA-HBF and DSA-HBF. In FC-HBF, all RF chains are connected to all the antennas, providing the maximum degree of freedom. This structure offers the best performance, but it requires a large number of PS that consume a non-negligible amount of energy. In the FSA-HBF structure, RF chains are connected to a fixed subset of antennas. The number of PS being reduced, the energy efficiency of this structure is higher than FC-HBF [3]}. Finally, in DSA-HBF, the connections between RF chains and antennas can dynamically change using low power multiplexers, as proposed in [4]}, and enable further energy-efficiency improvements.
